On the excitation of polarised light by electron impact

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In the course of some experiments on polarisation effects shown by mercury lines, emitted from a low-pressure electron-maintained arc, it was found that the yellow mercury lines λ 5770, 5791 are weakly polarised even in the absence of a magnetic field, the direction of the maximum electric vector being parallel to the direction of the discharge. From the general characteristics of the effect, it appeared likely that the polarisation is due to the partly directed character of the electron tracks in the arc; and, in this way, one was led to the view that an electron is capable of exciting an atom to the emission of polarised light. The present paper describes an attempt to investigate this point more thoroughly. While the work was in progress, two papers have appeared in which a search for signs of polarisation in the light excited by electron impact has been made. The first is by Kossel and Gerthsen who examined the case of the D lines of Sodium with a negative result. This was confirmed by Ellett, Foote and Mohler, who also investigated the case of the mercury line λ 2537 with a positive result, which will be described subsequently. These experiments, however, only dealt with a few individual spectral lines. In the present work data have been obtained for all the more prominent lines of the mercury Spectrum.